FERNANDEZ Diana Elizabeth
congresos y reuniones científicas
Xiphosurid trackways in a marginal marine setting from the Lower Cretaceous (Agrio Formation) of the Neuquén Basin, Argentina.
Sao Leopoldo
Congreso; Iº Latin American Symposium on Ichnology; 2010
Institución organizadora:
The Agua de la Mula Member of the Agrio Formation (Late Hauterivian-EarlyBarremian) is a mixed carbonate-siliciclastic, mainly marine and marginal-marine succession, exposed in the Neuquén province, Argentina. The bed containing the trackways (K level) corresponds to one of the uppermost levels of this unit, approximately 8 m below the contact with the Huitrín Formation, in the outcrops located at Cerro Rayoso. It is part of a 7.5-m-thick interval that begins with siltstones with poorly preserved desiccation cracks, followed by a thin heterolithic interval with organic debris, limestones having desiccation cracks, and a sandstone bed with abundant Rhizocorallium. This interval is followed by a sandstone (K level) containing very well-preserved ripples. These have mostly sinuous crests, but in some parts they tend to be unconnected, forming linguoid ripples. Some are flat-topped ripples, reflecting subaerial exposure. The dominant eastward paleocurrent  reflects an onshore flow direction. The xiphosurid trackways are preserved as negative epireliefs, recording surface impressions and shallow undertracks . They are assigned to Kouphichnium Nopsca 1923. The trackways are highly abundant and display a number of preservational variations depending mostly on their orientation with respect to the ripple crests and undertrack fallout. They are mostly preserved on the crests and are parallel to them. In some cases, they climb up and down the ripple crests and troughs. Telson impressions are conspicuous and continuous but not always present. Regular leg and "pusher" leg marks are common. Another ichnotaxa found in the K level are Gordia, Cochlichnus and ?Treptichnus; they were produced after the xiphosurid trackways and under a thin water film. These xiphosurid trackways represent the first known evidence of the existence of these traces in the Cretaceous of the Neuquén Basin.